Outflow Velocities in Polar Coronal Holes
Abstract
It is well established that the fast solar wind originates during the solar minimum from the polar coronal holes. The object is to investigate this wind onset region to identify the solar structures responsible. We report here the first measurements of outflow velocities between 1.0 and 1.3 Ro in solar plumes using XUV spectral lines from SUMER/SOHO and the Doppler dimming technique. In contrast to what has been suggested by several other observers using SOHO data we conclude that the wind velocity in plumes is greater than in the interplume regions. This tendency diminishes with height so that it may vanish or even reverse at greater distances. We show that one half of the solar wind observed at 1 A.U. from Ulysses originates from the solar plumes at 1.1 Ro. We are extending these observations to 4 Ro using the UVCS/SOHO spectro-coronagraph to understand the connection with plumes seen at greater distances. Initial indications suggest a change in the physical nature of plumes around 2.0 Ro raising questions concerning their relation to the frequently seen white-light plumes at large distances. We explore the possible connection between polar plumes the supergranule network and coronal heating in non-hole regions.
- Publication:
-
Stars as Suns : Activity, Evolution and Planets
- Pub Date:
- January 2004
- Bibcode:
- 2004IAUS..219..635G